The present invention relates generally to human or animal heart valves, and more particularly, to a heart valve prosthesis suitable for use in humans and adapted to provide more reliable service in use than prior art counterpart valves.
A typical artificial heart valve assembly of an accepted type includes a main body member of annular form, means for securing this body in a desired position within the heart, and one or more valves adapted to control flow of blood through this passageway. While the typical prior art valve assemblies have included ball check type valves, and disc or reed type valves, the most commonly accepted valve used today is the so-called leaflet type valve.
In such constructions, two leaflets are normally preferred and are disposed in opposed or mirror image relation. In the closed position, each valve leaflet occludes or covers half of the opening, with each leaflet being roughly semi-circular in shape and having a rounded exterior margin and an edge portion which engages an inner surface of the main body to provide a peripheral seal, and an inner, diametrically extending edge and adjacent margins adapted to abut the counterpart edges and margins on the second leaflet. Each leaflet is positioned for a rocking or pivoting movement about an axis extending parallel to its interior transversely extending edge surface and offset slightly therefrom.
In use, as blood pressure rises in response to heart contraction or systole, the leaflets are pivoted from a closed position to an open position to permit blood to flow past the leaflets, whereas when the heart contraction is complete, blood tends to flow in the opposite direction in response to the pressure in the aorta, causing the leaflets to close and maintain a pressure in the arterial system. The valve thus operates in the same manner as would be the case with a normal human heart valve.
A number of significant improvements have been made in heart valves, some of the most significant being improvements in materials, including the improvement which comprises coating all exterior surfaces of the prosthesis with pyrolytic carbon. A typical method of coating pyrolytic carbon onto a valve substrate is disclosed in U.S. Pat. No. 3,526,005. This provides very hard surfaces for the components to reduce wear, and provides some insurance against formation of an adhesion of clots to such surfaces.
In spite of significant advances which have taken place in the construction of heart valves, however, there is still room for significant improvement in the area of what may be termed operational reliability.
As is well known, even properly functioning mechanical devices are subject to eventual wear in use. Where the wearing is of a type which may lead to improper functioning or catastrophic failure during a time less than the anticipated life span of a patient, in the case of heart valves, such design is considered as being at least potentially unsatisfactory. Consequently, studies of the durability and reliability of heart valves have been undertaken, and these indicate the potential for further design improvement.
Where the wearing of movable parts is confined in a single, known location, it is anticipated that failure may occur in this location well before it would occur elsewhere in the device. With the now widely accepted simplified single or double leaflet type of heart valve just described, there are only one or two moving parts, and these move with respect to a single, fixed element.
The most widely accepted type of heart valve presently mounts its leaflets for pivoting movement by means of rounded ears extending radially outwardly from sidewalls of the leaflets in an area spaced just apart from the diametrically extending surfaces of the leaflet. These ears are received within guideways which are in the form of contoured grooves extending radially into the main valve body which, as described above, is a generally cylindrical or annular body. The guide surfaces or recesses are complementary in one respect to the shape of the ears. In the prior art, these have been formed as surfaces of at least partial revolution of circular or nearly circular arcuate surfaces. In other words, where the ear is formed as a portion of a circle having a given radius, the counterpart recess is formed as a surface having a slightly greater radius. With corresponding recesses formed in opposed sides of the valve body, and counterpart ears formed on opposite sides of the leaflets, the leaflet, when installed, pivots about the axis formed by these curvilinear, usually partially spherical surfaces.
The surfaces are precisely machined so as to provide a small but definite working clearance for the pivoting parts. When manufactured, the body is deformed or distended so that the ears may be inserted, and each unit so manufactured is then lab tested "dry" to ensure that the leaflets are held tightly enough to be secure against falling free from the prosthesis, but are not so tightly engaged so as to create a binding or restricted valve action.
In the use of this design, and other known designs, a significant potential problem has arisen. This problem has been the concentration of wear in certain areas so that the overall wear is not distributed evenly. In other words, while it had been anticipated that the wear on rounded or spherical ear surfaces and their associated recesses would be minimal, it has been discovered that instead, because of the large amount of play that can occur in the direction of the leaflet chord when the leaflet closes, wear is caused as the leaflet slides relative to the pivot line; the radially curved ear and recessed formations undergo localized wear because of this type of "slop" or sideplay clearance which can cause increased wear in the region where the pivot slides along the ear. This is a function of the degree of play, and is most pronounced when components at the extremes of the tolerance limits are used in a valve. Pivot point play also causes wear where a pivot point supports an ear surface of the leaflet.
As a result of this enhanced wear, while perhaps minimal by some standards, the motion sequence of the valve leaflets becomes less well defined, which in turn causes their motion to be characterized by excess play with respect to the pivot axes, enhancing the potential for binding and/or becoming released into the heart. This results in erratic performance on the one hand, usually manifested in asynchronous closure, or catastrophic failure, such as leaflet release or jamming, on the other hand.
The problem with the best known existing designs, therefore, has been one of an inability to precisely adjust the play and minimize it so malfunctions caused by improper fit-up do not occur. In this connection, it will be appreciated that since each unit is tested individually, manufacturing tolerances are sufficiently accurate to create whatever reliability potential may be achieved with the design. However, with localized rather than distributed wear occuring in existing designs, a difficulty has been created which is impractical to cure by imposing tighter manufacturing tolerances or different fit-up practices. The inherent inability to adjust and control the "slop" in the valves with circular tabs is considered a significant design defect.
In this connection, while prior art heart valves are reliable, and in many cases have been shown on the average to have a projected life expectancy exceeding that of the patient, these figures are statisical and therefore raise the possibility that there may be some erratic function or premature failures caused by extremes of free play in valve fit-up at the extremes of the allowable tolerance. An example is the asynchronous closure of certain prior art leaflet valves, for example.
Moreover, since it is almost always human lives which are being dealt with, there is a definite need for a prosthesis which will not just meet, but which will substantially exceed projected reliability and performance requirements.
In view of the need for further improvement of a heart valve prostheses, it is therefore an object of the present invention to provide an improved heart valve having a novel construction, including a novel method of forming a pivotable connection between the valve leaflets and the valve body.
Another object of the invention is to provide a heart valve having a pair of leaflets each received within a generallly cylindrical valve body, with each leaflet including an oppositely disposed pair of mounting ears of a particular geometric form and arranged for reception within counterpart recesses of complementary form.
Yet another object of the present invention is to provide a novel combination of mounting ear and recess formation adapted for use in single or double leaflet heart valves and adapted to provide greater reliability in use and generally improved functioning.
A further object of the invention is to provide a heart valve construction which enables the valve to be made from synthetic materials and entirely of or coated with pyrolytic carbon, and which is also able to provide a carefully controlled pivoting action as well as one in which whatever wear does occur will not be detrimental to the operation of the leaflets.
Another object of the invention is to provide a heart valve prosthesis which has the advantages of prior art heart valves, but which is free from the drawback of potential lack of reliability and reproducible function associated with such prior art valves.
Yet another object of the invention is to provide a heart valve leaflet which is of a generally semicircular form, and which includes opposed pivot or mounting ears formed on chordal portions of the valve leaflet lying at or near either end of its circular sidewall portions, with each of the ears, in plan view, being of a generally trapezoidal form, and having edges formed as parts of a frusto-conical surface.
The foregoing and other objects and advantages of the invention are achieved in practice by providing a heart valve having a cylindrical main body portion with valve mounting means lying on an inner surface of the body, with the body further defining a central passage for the flow of blood, and including at least one leaflet adapted to move between a closed position occluding blood flow therethrough and open position permitting blood flow therethrough, and being pivoted about an axis of movement which extends along a generally straight line transverse to the cylindrical axis of the body, and with the leaflet including mounting pivot ears of generally trapezoidal plan form. The leaflet mounting recesses are of a particular size and shape to provide improved functioning in use.
The exact manner in which the foregoing and other objects and advantages of the invention are achieved in practice will become more clearly apparent when reference is made to the following detailed description of the preferred embodiments of the invention set forth by way of example and shown in the accompanying drawings, wherein like reference numbers indicate corresponding parts throughout.